The rivers of Africa and South America are full of shocking conversations. Both continents are home to fish that can talk to each other using electric fields: the elephantfishes of Africa, and the knifefishes of South America (including the famous electric eel). Both groups live in dark, murky water where it’s hard to see where you’re swimming. Both have adapted by using electricity to guide their way. Their bodies have become living batteries and their muscles can produce electric currents that help them communicate, hunt, navigate and court.

But both elephantfishes and knifefishes evolved their electric powers independently. Their common ancestors had no such abilities. They are a great example of how two groups of animals, faced with a similar problem, can arrive at the same solution. And this similarity is all the more striking because it is based on the same gene. For a fish, it seems there are only so many ways to be electric.

Matthew Arnegard from the University of British Columbia has found that the shocking skills of both elephantfishes and knifefishes depend on a gene called Scn4a. The gene produces proteins that sit on the surface of muscle cells and act as gates. When they are open, they allow positively charged sodium atoms to cross the cell’s boundaries, causing the muscle to contract. This happens whenever we move, whenever our muscles flex. How do you go from such an ordinary event to actually producing electricity? It turns out that it’s all in the timing.

Electric fish produce currents using special cells in their tails called electrocytes, which are stacked in rows like the cells of a battery. Electrocytes are close relatives of muscle cells, and they too are laden with sodium-channelling gates. The big difference is that all of their gates open at the same time. This simultaneous ‘firing’ produces a very strong voltage. In the electric eel, it’s strong enough to kill; in other weaker species, it’s enough to communicate with. Both elephantfishes and knifefishes produce electricity in the same way.

Scn4a’s role in all of this makes sense. It produces gates that are an important part of both contracting muscles and firing electrocytes. But there’s a problem with this story – Scn4a is so important that it’s not a gene that can be lightly tinkered with. When people inherit a faulty copy, they tend to develop severe muscle disorders that prevent them from relaxing their grip or walking properly, and sometimes even paralysing them.

Fish, however, have got round that problem. Evolution can tinker with their Scn4a all it likes because fish, through a fluke event, have two copies. A few hundred million years ago, an ancestral fish somehow ended up with two copies of its entire genome, doubling the number of genes that its descendants would inherit. These duplications often happen and they provide raw fuel for evolution. By creating back-up copies of important genes like Scn4a, they allow the originals to be tweaked with few consequences.

Modern fish actually have two related copies of Scn4a, catchily named Scn4aa and Scn4ab. It’s the former that both groups of electric fish have independently co-opted to produce their chatty currents. Arnegard studied many species of electric fish and found that, as they grow up, Scn4aa is hardly used in their normal muscles but strongly activated in their electrocytes. That’s true of both elephantfishes and knifefishes, but not of their relatives without electric powers.

Arnegard also looked at how the gene changed as these fish evolved. After the electric fish started to diverge from their static relatives, their copies of Scn4aa started to change rapidly. Not only that but these changes were major ones; they affected the very structure of the gate protein that the gene codes for. In both electric dynasties, Scn4aa was evolving fast. By contrast, its counterpart Scn4ab showed no signs of such dramatic changes.

Of course, the vast majority of fishes have two copies of Scn4a, so why did only the elephantfishes and knifefishes put one of their copies to use in producing electricity? You could argue that both these groups had a clear need for such an ability because they live in murky water, but so do many other fish.

Rather, Arnegard thinks that the real reason is that both of these groups had already evolved to sense electric fields. From there, it was a simple step towards actually generating the fields themselves. In both cases, Scn4aa was the right gene for the job – the back-up gene that allowed two groups of fish to find the same new way of chatting in the darkness. It has certainly been a successful strategy for them. Today, there are around 150 species of knifefish and 200 species of elephantfishes.

This somehow confirms the importance of what was called “junk DNA”: raw genetic material to play with, in order to acquire new functions. Redundancy allows system robustness, which in turn speeds up evolution!

Since both gene copies remained functional, neither would have been considered to be “junk” DNA. It does share a mechanism of origin with some junk DNA, which arises from gene duplication events with subsequent function-destroying mutation of some of the copies.

But the loss of function mutation is most often irreversible. I think there are a few cases a sequence of previously non-functional DNA regained function or was co-opted for a new function, but these are very rare exceptions to the rule.

BTW I see a similarity between the two, because both are “not essential” DNA and they’re not constrained by the same selective pressure of a one-copy functional gene. This way they have the potential to mutate and create new genes or new regulatory functions.

A recent paper (which was commented by Ed himself ) suggests that the evolution of complex life forms was driven by energetic availability, which allows to maintain big genomes and to experiment with them.

My comment was just a thought about the opportunities raised by having more non-essential DNA.

Indeed. Several writers have referred to this sort of DNA as “fossil DNA”, as it provides a record of an organism’s evolutionary history. This would apply for both the non-functional and the functional duplicated genetic material.

I sometimes wonder whether or not all genes arose originally through duplication – ie is there a universal common ancestral gene, from which all surviving modern genes are descended from? Or is it more likely that there were a collection of such ancestral genes, each with its own lineage of descent all the way back to the first self-replicating molecules that stored information. Probably the original versions of all these genes would not have been DNA (they would have had the same information stored in a different molecular format, transmitting that information to descendent DNA at whatever point and by whatever process that resulted in the change to DNA for the genetic material).

I’m not sure if this question of ultimate gene ancestry is answerable by current techniques, or even answerable in theory. I suppose time will tell.